The present invention relates to a new and improved method of starting-up the casting of a strand during continuous casting, wherein molten metal, typically steel, is cast or teemed through a bottom pouring opening of a tundish at a throughflow rate essentially governed by the dimensions of the bottom pouring opening, into a continuous casting mold, and there is formed by the dummy bar head of a dummy bar a connection between such dummy bar and the solidified strand and upon reaching a predetermined bath level in the continuous casting mold the strand is withdrawn. Further, the invention relates to a new and improved construction of apparatus for the performance of the aforementioned method.
During the continuous casting of steel and as a general rule when forming billet-and bloom-shapes, there are employed tundishes having open bottom pour openings. With such open bottom pour openings, which are neither equipped with a slide-nor a stopper-control, the throughflow rate of the cast metal can be influenced by different degrees of filling of the tundish. A further factor which affects the throughflow rate is the viscosity of the metal which is influenced by the casting temperature. However, the throughflow rate is essentially determined by the dimensions of the pouring outlet or opening.
During the start of casting a strand the mold outlet is closed by the dummy bar head of the dummy bar. This dummy bar head, which for instance can be a permenant dummy bar head, forms a connection with the solidified strand. The contemplated casting speed essentially determines the dimensions of the bottom pouring opening. Also resulting therefrom is the time needed to fill the mold during the start of casting. If a pouring outlet nozzle is provided for instance for a mold having the dimensions 200 mm.times. 200 mm for 600 kg steel per minute, i.e. for a casting speed of 2 meters per minute, then between the start of casting the steel into the mold and the beginning of strand withdrawal, for a filling height in the tundish of about 100 mm and a filling height of the mold of 600 mm, there is available between 28 to 34 seconds. This time which is available for the solidification of a coupling piece at the hot strand and the formation of a strand shell or skin which safeguards against metal breakouts oftentimes is not sufficient to insure that casting can be started without the danger of metal breakouts. By inserting the emergency launder the possibility exists of interrupting the casting operation for such length of time until there has been obtained the desired solidification. The insertion of the emergency launder is, however, not only quite dangerous due to the rather pronounced spraying of steel but furthermore the entire continuous casting installation is contaminated by the steel spatters. Additionally, the danger exists that the emergency launder will already have been soiled when real emergency situations arise and therefore will be impaired with respect to its functional reliability.
The drawbacks of the shorter starting casting times can be overcome by selecting a bottom outlet nozzle having a smaller diameter. By virtue of this measure there is however simultaneously reduced the speed and therefore the output of the continuous casting installation so that, for instance, it is no longer possible to carry-out sequential pours or to cast with large ladle capacities.
In order to maintain constant the outfeed rate even with decreasing fill height and decreasing temperature of the cast metal, it is known during block casting to employ casting nozzles having an inner cross-section which reduces in the direction of the outlet end. The casting nozzle protrudes in relation to the casting vessel, so that the length of the nozzle can be shortened by cutting-off parts thereof and thus there can be enlarged in a number of steps the outlet cross-section. The cutting operation is carried out by means of a clamp or pliers which are applied to pre-formed notches. During cutting it is impossible to avoid that some refractory particles will be flushed by the casting jet into the cast piece. When applying this technique for the continuous casting of strand such refractory particles will be washed directly into the metal bath of the continuous casting mold where, especially in the case of small cross-sections, they will become frozen in the strands shell or skin and can cause strand defects or breakouts. Due to breaking-off of a nozzle part there also will be damaged the edge of the nozzle mouth, disturbing the formation of a faultless casting jet and causing the known drawbacks.
Further, there is known to the art a pouring opening or outlet for casting ladles having an outflow quantity which changes as a function of the pouring or outflow time and which possesses an outflow channel widening in the outflow direction. The outflow channel is subdivided into a number of individual sections which, in the outflow direction, possess from section to section increased wear resistance of the refractory material. With such type pouring opening it is possible to achieve an automatic regulation within certain limits without the need for external action, especially when carrying out a casting operation under the exclusion of air. When using such outlet nozzle or opening at a tundish in a continuous casting installation refractory particles are flushed into the mold due to the errosive removal of the nozzle material, and thus the melt is contaminated. On the one hand, there is thus impaired the quality of the strand and, on the other hand, flushed-in refractory constituents can cause metal breakouts. Moreover, such regulation is extremely sluggish and cannot be utilized for influencing the throughflow rate within short time intervals.